By way of examples, the circuits commanded by the command protocol according to the invention can be switching devices or else motors of motorized antennas.
As is known to those skilled in the art, the electromagnetic waves emitted by a telecommunication and/or telebroadcast satellite are horizontally or vertically polarized microwaves whose frequencies lie within a high-frequency band or within a low-frequency band.
At the receiving end, the signal transported at microwave frequency by one or other of the two abovementioned polarizations is transposed into the satellite intermediate band by means of local oscillators located in low-noise conversion blocks which will subsequently be denoted LNB (standing for Low Noise Block).
The satellite intermediate band, generally denoted SIB, covers the frequencies lying between 950 MHz and 2150 MHz. It is subdivided into two sub-bands generally termed the low band and the high band. The low band covers the frequency range lying between 950 MHz and 1950 MHz and the high band covers the frequency range lying between 1100 MHz and 2150 MHz.
A command protocol allows the reception devices to select the polarization of the wave as well as the frequency band which it is desired to receive.
In the case of the satellite telecommunication system known by those skilled in the art as the ASTRA system, the reception device consists of a receiving antenna, an LNB block and a decoder.
The receiving antenna makes it possible to gather the microwaves originating from telecommunication and/or telebroadcast satellites.
The LNB block makes it possible to amplify the microwaves received, to select their polarization and their frequency band and to transpose them into intermediate frequency signals within the SIB band.
The decoder demodulates the intermediate frequency signal from the LNB block in order to generate the video signal which it contains.
The function of the decoder is also to transmit various cues to the LNB block allowing the latter to operate correctly and, especially, to carry out the wave polarization selection function and the frequency band selection function. The cues transmitted from the decoder to the LNB block are generally transmitted by the same transmission device as that which transmits the intermediate frequency signal from the LNB block to the decoder.
The function for selecting the polarization of the wave is performed by varying a supply voltage output by the decoder.
A first switching device located in the LNB block makes it possible to command the choice of the polarization of the wave with the aid of a command voltage from the decoder.
For a command voltage value of between 12.5 V and 14 V the switching device selects the vertically polarized wave and for a command voltage value of between 17 V and 19 V, the switching device selects the horizontally polarized wave.
The function for selecting the frequency band is performed with the aid of a periodic signal output by the decoder and whose frequency f.sub.c is equal to 22 kHz. To this end, a second switching device located in the LNB block makes it possible to command the choice of the frequency band with the aid of the periodic signal of frequency f.sub.c which it receives.
In the absence of the signal of frequency f.sub.c, the switching device selects the low-frequency band and in the presence of the signal of frequency f.sub.c, the switching device selects the high-frequency band.
The ASTRA telecommunication system relates more particularly to the reception of signals by a single reception device.
If it is desired to construct complex installations such as, for example, installations allowing multi-reception (N satellites/M reception devices) new variables must be defined in order to implement the command protocol.
Thus, in order to manage the commanding of complex systems, it has been proposed to introduce, into the signal reception devices, a control facility commonly referred to as a DiSEqC and which will subsequently be referred to as a DiSEqC system (the acronym DiSEqC standing for "Digital Satellite Equipment Control").
The specifications relating to the DiSEqC system are described in the document "Digital Satellite Equipment Control (DiSEqC.TM.) BUS FUNCTIONAL SPECIFICATION Version 4.1" published on Feb. 6, 1997 by the European organization for satellite telecommunications.
The DiSEqC system makes it possible to generate cues in the form of "0"s and "1"s, each "0" or "1" being constructed with the aid of the abovementioned 22 kHz frequency signal.
Messages, consisting of combinations of "0"s and of "1"s are then used as parameters of the abovementioned command control protocol.
A command protocol using such a type of message has the drawback of being particularly slow.
By way of example, a switching device operating according to the DiSEqC mode makes it possible to switch from a first LNB to a second LNB in around 260 ms.
Such a control procedure duration is not compatible with the durations required, for example, by the programme broadcasters who prescribe a lock-on time for the functions of frequency conversion of the tuners and demodulation of the circuits located in the decoders of less than or equal to 100 ms.
Thus, according to the prior art, the management of complex installations proves to be virtually impossible in the time required.
The invention does not have this drawback.